As a known method for forming a visible image on a recording material from an electrical image signal, the method called xerography is generally used. In this method, first, an electrostatic pattern (electrostatic latent image) is formed on a visualizing member including a photoconductive layer having an electro-optical property by optical writing means, and by making visualizing particles (hereinafter referred to as toners) adhere onto the electrostatic pattern, the static pattern is visualized. Thereafter, the toner on the visualizing member is transferred onto a recording material, thereby visualizing the image signal onto the recording material as a visible image.
More concretely, using a light emitting device (element) such as a laser, LED (Light Emitting Diode), etc., an image signal is converted into an optical signal, and the light beam is projected on the photoconductive layer having been uniformly charged beforehand, thereby forming an electrostatic pattern in accordance with the light intensity on the photoconductive layer. Then, charged toner is made to adhere to the electrostatic pattern or trajected so as to form a toner image on the photoconductive layer. (The process is hereinafter referred to as a developing process).
Then, the toner on the visualizing member is sucked onto the recording material electrically and or under pressure. (The process is hereinafter referred to as a transfer process). Therefore, under an applied pressure and/or heat, the toner image is made permanent on the recording material.
Other than the described xerography, an image may be formed using a dielectric drum (visualizing member), a charged particle generator and a charged particle flow control grid. In this method, by controlling the voltage to be applied to the charged particle flow control grid according to an image signal, the flow of the charged particle generated from the charged particle generator is controlled. As a result, a charge pattern based on the image signal is formed on the dielectric drum. Thereafter, the charge pattern is developed using the toner, thereby forming a toner image on the dielectric drum. Then, the toner on the dielectric drum is transferred onto a recording material and is made permanent thereon in the same manner as the previous method.
In the described two image forming methods, the recording material is recognized mainly using a limit switch, etc., and whether or not the recording material has passed is detected under a control of the mechanical contact state using a transportation force from the recording material. Moreover, in order to detect the width of the recording material, a sensor is required separately.
However, in order to detect whether or not the recording material exists using the limit switch, since components including a sensor, etc., are required for detecting the width of the recording material,i the problem is presented in that a greater number of components is required. Moreover, in the case of using the limit switch, since the detection is carried out under a control using the mechanical contacts an operation error due to contact inferiors is likely to occur.
Other than the method using the limit switch, a method using an electrostatic capacity for detecting whether or not the recording material exists and detecting the size of the recording material has been proposed. The method is disclosed in Japanese Laid-Open Patent Publication No. 260943/1985 (Tokukaisho 60-260943). This method is applicable to a detector. However, when the method is applied to the detector, an electrode which covers an area of the recording material is required.
Recently, a method for directly forming a toner image on the recording material without using the visualizing member has been proposed. In this method, using the charged particle flow control grid controlled based on the image signal, the charged toner is selectively and directly trajected onto the recording material so as to form the toner image on the recording material. Thereafter, the toner image is made permanent on the recording material in the same manner as the previously described methods.
In this method, since the visualizing member for forming the electrostatic pattern based on the image signal can be eliminated, a simplified structure and compact size of the image forming apparatus can be achieved.
However, when forming an image without using the visualizing member, physical properties (thickness, dielectric constant and changes in these properties due to environmental changes, etc.,) of the recording material will affect the trajection of the toner. Therefore, as in the described method, the detection by the limit switch, the detection determining whether or not the recording material has passed based on changes in electrostatic capacity, the detection determining the width of the recording material using the sensor, etc., are not sufficient to recognize the recording material. Therefore, the problem is presented in that a stable image quality according to the physical properties of the recording material cannot be ensured.
Another method is disclosed in Japanese Laid-Open Patent Publication No. 204149/1992 (Tokukaihei 4-204149), wherein an image forming operation is controlled by measuring the surface resistance of the recording material. However, in this method also, components including a sensor, etc., are separately required for detecting the width of the recording material, thereby presenting the problem of increasing the number of components.